AI problem at altitude

William Pete Hodges

William Pete Hodges

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Petehdgs
Hello POA,
I had an unusual occurrence on my last XC flight and thought I'd ask if anyone has seen this happen before. I make a few trips each year 375 NM in my 64 Mooney Super21. This last time on my return trip the AI stopped working about an hour into the flight. It showed about a 10 degree right bank angle when the airplane was straight and level. The vacuum gauge showed a steady 5.5 inches the whole time. I was cruising VFR at 9500 feet and about 140 kts TAS, 10 degrees C, and altimeter was 30.03 to 30.10 during the flight. After about 2 hours I noticed the DG seemed to be out of service as well. The vacuum gauge still read 5.5.

Now here is the funny part. When I entered the traffic pattern both instruments were again working fine. I talked to my mechanic and also another mechanic with aviation experience. They were unsure what would cause this problem and then reverse itself again at low altitude. One of the suggestions is to check all the vacuum hoses and connections on the instruments themselves. I intend to on my next visit to the airport.

I was wondering if any of you have ever had a similar problem and what did you do to correct it?

Thanks in advance for your consideration.
 
Yep, replaced the AI.

With both happening, there's an issue with the vac system that isn't affecting the gauge. Bet there's a leak upstream/downstream(?) of the gauge, or in one of the instruments. Disconnect one, attempt to duplicate. Repeat for other.
 
‘Stopped working’, but meaning had the 10 degree list? Recaging no help? ‘DG out of service’? Can we trust the vacuum gage? In the short term I would just fly VFR, evaluate, & stand by to use backup methods.

Maybe time for the two G5’s?
 
Biggest problems with vacuum systems are often the filters that never get changed. Depending on how that vacuum gauge is plumbed, it might read fine but little airflow is going though the instrument, especially at altitude where the air density is less. The gauge should be plumbed across the air inlet and vacuum lines to get a true reading of the differential across the instruments. If the gauge's air port is just left open and the vac port is connected, a clogged filter will stop airflow but the gauge will indicate properly.

There's a central air filter that cleans the cabin air before it's sucked into the instruments. There's normally a foam garter filter on the vacuum relief valve (regulator) that filters the air let in by the relief to moderate the vacuum pressure. I have found those foam filters crumbling with great age, which contaminates the valve and lets garbage into the vacuum pump. Some old vac regulators had just a screen (up to mid-1960s, maybe). They were often on the firewall and got all sort of junk plugging that screen). Central filters get plugged by the tar from cigarette smoke, as well as dust.

This is all annual inspection stuff. But it's hard to get under some panels to inspect, so.....
 
I agree with Dan, clogged filter reducing the flow, and critical flow too low at altitude. Our gyro went stupid first, followed by DG, just like this example.

Our avionics shop replaced our small filter with one twice as large for a twin using more gyro instruments, and we were fine. After we outlawed smoking in the plane, filters were never a problem again.
 
How many hours are on the instruments? Could be they are both worn out.
 
geezer said:
I agree with Dan, clogged filter reducing the flow, and critical flow too low at altitude. Our gyro went stupid first, followed by DG, just like this example.

Our avionics shop replaced our small filter with one twice as large for a twin using more gyro instruments, and we were fine. After we outlawed smoking in the plane, filters were never a problem again.
Click to expand...

Smoking is an issue requiring more frequent filter changes, so is hauling dogs or other pets not in proper carriers.
 
If you position the Relief Valve that has a screen with the screen on the

bottom less crud will accumulate.

Gravity sucks!

I believe your aircraft originally used the Vacuum System for Step Retraction

and autopilot servos as well.

When seals and hoses cracked or servo filters dirtied up you could

have issues.

Generally low vacuum though so not the case here.
 
Magman said:
If you position the Relief Valve that has a screen with the screen on the

bottom less crud will accumulate.

Gravity sucks!

I believe your aircraft originally used the Vacuum System for Step Retraction

and autopilot servos as well.

When seals and hoses cracked or servo filters dirtied up you could

have issues.

Generally low vacuum though so not the case here.
Click to expand...

My '64 Super21 M20E has manual step retract and manually operated speed brakes. It does not have the Brittain vacuum operated wing leveler, but does have an all electric legacy auto pilot. The AP works ok in warm weather but not in cold. I think the electro-mechanical gyro sensor needs overhauling, but up to now I have been occupied with more pressing issues. I fly VFR, some long XC, and moderately short fields and grass. (2E8 7W4 W75 TGI VG18)

I am into my 3rd year of ownership with this aircraft. The first upgrade (year1) was adding ADS-B capability with a Garmin GTX345 and a panel mounted AERA660. Then I added a multicylinder engine monitor from another M20 aircraft and took the advanced engine management classes.
At the end of last summer nearly all of my engine instrument cluster had failed, and I was having problems with my used multi cylinder engine monitor. So after weighing the options I took my mechanic's advice and did a partial panel rework and installed an AI CGR30P engine monitor panel that replaced all the ailments at once. The AERA660 was upgraded to the AERA760 after negotiations with Garmin (that is another story).

All the filters in the Vacuum System have been replaced within the last 3 years. I haven't checked the log yet but the AI and DG are both older and it may be time for overhaul. So it sounds like I'll be sending them out along with the AP gyro at my next annual in October.

One Miracle at a time.
 
Should be able to swap in a replacement for almost free. Just like the older transponders, vacuum attitude gyros should be easy to find.
 
Both gyro's getting woozey at the same time points outside the gyro itself.

EDITED TO ADD
The filters that I have serviced were multilayer, clothe between wire screens to mechanically contain and protect the soft filter clothe.

Simply cleaning the screen is time and effort wasted. Our A/P did just that when we complained about gyro weakness, and then recommended gyro replacement. We went to the avionics guy, he showed us the solidly clogged clothe, and sold us the larger filter. Both gyros continued to give good service for years after.
 
Last edited:
Doug Reid said:
The gage shows vacuum pressure, not rate of airflow. I agree with Dan.
Click to expand...
Depends on the gauge's plumbing, as I mentioned. If it's properly done, the gauge measures the differential pressure between the vacuum port and the air inlet port of one of the instruments. The instruments are plumbed in parallel, so if the differential is at the proper pressure (4.6-5.4"Hg) they're getting adequate airflow. If the central filter gets plugged, airflow diminishes and the differential disappears and the gauge reading falls. You have to have airflow to get a differential.

But if that gauge is plumbed like some I have found, only the vacuum port is measured and the gauge's air port is left open to the cabin. Now it's measuring absolute vacuum instead of differential pressure, and the gauge will still read normally if the filter is plugged. In the correct plumbing, when airflow stops, both ports get vacuum (one directly from the vac line and the other through the instrument and the gauge reads zero. Pascal's Law.
 
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